Project Fox Driveshaft - Getting Shafted
There's More Science Behind Driveshaft Dynamics Than Meets The Eye. With The Help Of Strange, We Spec Out The Perfect Shaft For Project Fox.
From the June, 2010 issue of Popular Hot Rodding
By Stephen Kim
Photography by Stephen Kim
Whether you're transplanting a big-block into a small-block chassis, or an LS1 into a wimport, ordering up a custom driveshaft is a must for any engine swap project. In addition to changing the distance between the transmission and rearend, engine swaps bring hundreds of extra horsepower to the party. As one of the most highly stressed components in the entire driveline, if the driveshaft snaps in half, you'll be putting down exactly 0 hp to the rear wheels. How's that for parasitic driveline loss? Moreover, increases in horsepower, vehicle weight, and tire grip exacerbate the stresses that the driveshaft endures. With a 775hp big-block, a drag-style four-link, and sticky Mickey Thompson meats, our '93 Mustang project car is guilty of all three offenses.
For assistance in solidly linking together Project Fox's stout Phoenix TH400 trans and built 8.8-inch rearend, we called up Strange Engineering for some advice. After taking a few quick measurements and discussing the needs of our particular application with Strange, they got busy building us a custom 3-inch chrome-moly driveshaft and had it on our doorstep in less than a week. From our discussion with Strange, it became very obvious that a driveshaft is more than a simple piece of round tubing spinning inside a trans tunnel. There's actually quite a bit of science involved when it comes to minimizing vibrations, and maximizing strength and durability. Selecting the right driveshaft for any application requires understating how tubing diameter, material, and wall thickness-in addition to U-joint and yoke design-all impact driveshaft dynamics and strength. Rather than merely stabbing our new Strange driveshaft into Project Fox and marveling in our ability to pull off such a challenging feat, we'll take a crack at explaining the technical aspects of driveshaft design.
While the car was on the lift, we figured it was also a great opportunity to set up the pinion angle. The procedure for dialing in pinion angle differs depending on rear suspension design, but nonetheless, it can be accomplished with a few simple handtools and is critical for keeping rearend wrapup in check under acceleration. As always, thanks to the good folks at Bill Buck Race Cars in Austin, Texas, for helping us out.
Measuring for a driveshaft...
Measuring for a driveshaft is an easy two-step affair. The first step is recording the distance from the lip of the tailshaft housing to the front of the pinion yoke. The second measurement should be taken from the lip of the tailshaft housing to the end of the output shaft. Measurements should be taken with the car on the ground, or with jackstands placed underneath the front and rear suspension. We did just that, but later placed the Mustang on the lift for illustrative purpose.
Factoring in the length of...
Factoring in the length of the trans slip-yoke, the prior measurements help determine the correct center-to-center distance between the driveshaft's U-joints, which in our case happened to be 47 1/8 inches. Since Fox Mustangs aren't blessed with particularly commodious trans tunnels, Strange set us up with one of its 4130 chrome-moly 3-inch driveshafts, which is still plenty strong for our application. The tubing has a wall thickness of 0.083 inch, and Strange limits runout to 0.008 inch to ensure concentricity throughout the length of the shaft. At 22 pounds, it isn't exactly light, but we felt the bulletproof ruggedness of a chrome-moly shaft was worth the weight penalty.
Many factory driveshafts came...
Many factory driveshafts came equipped with 1310-series U-joints, which become marginal at around 450 hp. Strange uses larger 1350-series joints on its shafts for increased torque capacity. A 1310 U-joint measures 31/4 inches wide, while a 1350 joint measures 3 5/8 inches wide. Furthermore, 1310 joints have bearing cap diameters of 11/16 or 1 1/8 inches, while 1350 joints have larger 13/16-inch caps.
Strange offers transmission...
Strange offers transmission slip-yokes for every conceivable application in both mild steel and 4130 chrome-moly. They set us up with a chrome-moly unit to mate up with Project Fox's TH400, which has been heat treated for improved tensile strength. Much like balancing a tire, all Strange driveshafts are spun up to high speeds on a balancer, and weights are welded into place at just the right spots to smooth out vibrations. While most stock driveshafts are only balanced to 3,500 rpm or so, Strange balances its shafts up to 7,500 rpm depending on their intended use. Excessive vibration leads to both breakage and parasitic power loss.
Pinion angle is simply the...
Pinion angle is simply the angle of the pinion yoke in relation to the driveshaft. The easiest way to measure it is by laying a magnetic angle finder down on the driveshaft. They're available at most hardware stores for about $15.
When determining the center-to-center...
When determining the center-to-center length of a custom driveshaft, Strange factors in an inch of runout for the trans slip-yoke. This allows enough freeplay to prevent the yoke from bottoming out on the tailshaft as the suspension moves through its range of travel. On the other hand, having too much runout can cause insufficient spline engagement.
Like GM A-bodies, Fox Mustangs...
Like GM A-bodies, Fox Mustangs use a triangulated four-link rear suspension. With this arrangement, pinion angle is changed by shortening or lengthening the upper control arms until the angle finder gauge points where you want it. Since the control arms on Project Fox's Competition Engineering suspension have Heim joints at each end with very little freeplay, Bill Buck set the pinion angle at -2 degrees. One caveat on a triangulated four-link is that the upper control arms also locate the rearend from side to side, so the left and right links must measure roughly the same length to properly center the rearend. To adjust pinion angle on a leaf-spring car, shims must be wedged between the leafs and the rearend spring perches.
Ideally, a driveshaft would...
Ideally, a driveshaft would be positioned at a 0-degree angle, or parallel to the ground, but this isn't possible due to the packaging limitations of production car-based chassis. Such an arrangement allows both U-joints to operate at the same velocity, which minimizes wear, vibration, and parasitic power loss. As soon as any angle is introduced into the driveshaft, the U-joints begin traveling in an elliptical path instead of a circular path, and cause vibrations. Improperly phased U-joints make this condition dramatically worse, so it's imperative for driveshaft manufacturers to position the front and rear U-joints in line with each other.
The rpm at which a driveshaft becomes unstable is referred to as its critical speed. This instability causes a driveshaft to bend in the center like a jump rope, and prolonged operation at critical speed will eventually lead to parts failure. The formula for calculating critical speed is extremely complex, but suffice it to say that it's a function of driveshaft diameter, length, wall thickness, and the modulus of elasticity of the material it's made from. Generally, the shorter the length and the larger the diameter of a driveshaft, the higher its critical speed will be. Although there isn't much you can do about the length of driveshaft your application requires, high-performance aftermarket driveshafts are commonly available in 3-. 3.5-, and 4-inch diameters. The bigger the better, but there is a practical limit to how large you can go due to trans tunnel clearance. As far as driveshaft material is concerned, carbon fiber offers the highest critical speed, followed by aluminum, and then steel.
While critical speed is indicative of potential driveshaft failure due to prolonged high-speed operation, it doesn't necessarily reflect the strength of a driveshaft. The sheer abuse a driveshaft can handle is primarily attributable to the tubing material. The typical mild steel driveshaft used in many production cars can fail at power levels as low as 400 hp. High-performance aluminum driveshafts are extremely popular upgrades for muscle car enthusiasts due to their high strength and low mass, as they can survive loads up to 1,000 hp. The strongest material by far is DOM chrome-moly, which is often the choice of extreme-duty drag cars producing in excess of 2,000 hp. This strength comes with a weight penalty, however, which also increases parasitic driveline loss. Carbon fiber is the wild card of the lot. Some people claim that carbon-fiber shafts can support 800-plus horsepower, while others have reported failure at substantially lower power levels. Furthermore, while carbon fiber weighs next to nothing, it can also cost twice as much as a comparable chrome-moly driveshaft.
If your chassis is already hooking up hard out of the hole, chances are that there isn't much to be gained by changing the pinion angle. In essence, dialing in the right amount of pinion angle prevents a loss of traction rather than enhancing traction. As the driveshaft applies torque to the ring gear, it forces the top of the rearend housing to rotate rearward, and the bottom to rotate forward. If viewed from the passenger side of the car, the rearend naturally rotates counterclockwise under acceleration. Excessive rearend wrapup can unload the rear suspension, compromising grip. Pointing the pinion downward in relation to the driveshaft-also known as negative pinion angle-compensates for this effect. Having the right amount of pinion angle can prevent a loss of traction, but excessive amounts won't improve grip, and increases U-joint wear and parasitic driveline loss. "More negative pinion angle doesn't always give you extra bite, and how much angle a car needs depends on the suspension setup. The goal is to have the pinion in line with the driveshaft under acceleration, which requires dialing some negative pinion angle in when the car is in a static state," Bill Buck says. "With a stock suspension that uses rubber bushings, it might need as much as -7 degrees. Leaf-spring cars have more suspension play, so they need more angle than cars with control arm-style suspension. Cars with urethane bushings need about -4 degrees of angle, while cars with Heim joints need -2 to -3 degrees. An extreme example is Mike Murrillo's Outlaw 10.5 Mustang. Everything is so solidly linked in that car that there's hardly any axlewrap, which means it only needs -1 degree of pinion angle."
|WHERE THE MONEY WENT
|Strange chrome-moly driveshaft
|Strange trans yoke
|THE COST SO FAR
|'93 notchback Mustang
|Sold old wheels, tires, engine, trans
|532 big-block Ford
|Phoenix TH400 trans
|Strange 8.8 rearend
|Comp Engineering rear suspension
|AJE front suspension
|Bill Buck custom 10-point cage
|Engine and trans install
|Russell fuel system